UNIT: CONCRETE/MASONARY Concrete Proportions
MIXTURE OF INGREDIENTS • CEMENT PASTE – This is the mixture of cement and water that determines the concrete’s strength; it varies according to water-cement ratio. • Water-Cement Ratio a. These are usually 5, 6, or 7 gallons of water to 1 sack of cement; the most common ratio is 6:1 b. This amount of water takes into account the water in the sand. 2. Cementcomes in 94 pound sacks containing 1 cubic ft.
Sand has some water attached to its particles. This moisture needs to be estimated and an allowance made for it. The more moisture there is in the sand, the less water needs to be added when mixing.
Damp Sand falls apart when squeezed into a ball. It contains about ¼ gallon of water per cubic foot. • Wet sand forms a ball when squeezed but leaves no noticeable moisture on the palm. This is the most usual kind and contains about ½ gallon of water per cubic foot. • Very wet sand forms a ball when squeezed and leaves moisture on the palm. It contains about ¾ gallon of water per cubic foot.
Ratio of Cement:Fine Aggregate(Sand) : Coarse Aggregate (Gravel) Ratio is expressed as a three digit number called a mix, for example, 1:2:3. • A mix means 1 part cement, 2 parts sand, and 3 parts gravel. • The mix proportions can be based on either wt. Or volume.
Mix Proportions • A ratio of 1:2:3 can be stated 1 cu. Ft. (sack) of cement, 2 cu. Ft. of sand, and 3 cu. Ft. of gravel • It can also be 100 lb. (approx. a sack) of cement, 200lbs. Of sand, and 300lbs. Of gravel (1 cu. Ft. of aggregate is approximately 100lbs.)
A Correct Mixture assures that: • Each particle of sand and gravel is covered with cement paste. • Each particle is bound to other when the cement paste dries and gardens. 3. The intended use of the concrete also determines the ratio of the ingredients. Concrete Mixer
Coarse Aggregate Size • Should not exceed 1/3 the slab thickness • Should not exceed 1/5 the wall thickness • Maximum size is 1 ½ inch with the most common being ¾ inch.
Mixture Yield • Generally, the volume of mixed concrete will be about 2/3 the combined volume of the items used in the mix. • Examples of how much material should be used to make 1 cubic yard of concrete can be found in charts in any basic book about concrete.
Proper mixture of concrete being leveled on a slab. This will be used for a cattle shoot.
The Slump TestA rough measurement of concrete’s consistency and workability. Common slumps used for most jobs are 3-4 inches. Slump should never exceed 6”. The slump is determined by a test using a conical cylinder and a portion of the concrete batch to pour. The cylinder has a 4” diameter opening at the top and an 8” diameter opening at the bottom, and is 12” high.
Steps to Slump Test • Place moistened cone-shaped container on a flat, level surface. • Fill the container 1/3 full with sample of concrete. • Rod this layer 25 times using jabbing strokes with a 5/8” solid steel rod – jabbing strokes as this helps to settle the concrete.
Slump test steps continued… • Add a second layer until the container is 2/3 full. • Rod this layer an additional 25 times. • Finally, fill the cone to the top edge and level it. • Rod a final 25 times. Remove any spilled concrete from around the base of the cone. • Lift the cone off the rodded batch within five seconds.
More about slump test steps… • Without a retaining device the concrete will slump or fall from the original 12” height. • The difference in the original height and the final height is known as the slump.